AC-DC converter is used to convert an input AC voltage into an operating DC voltage for driving a DC load, such as a magnetron used for a microwave or a gas discharge lamp.
The conventional AC-DC converter generally includes a main switch circuit, a transformer, a resonant capacitor, and a switching control circuit. The resonant capacitor is connected to the current-conducting terminal of the main switch circuit, and constitutes a quasi-resonance circuit with the magnetizing inductance of the transformer. When the main switch circuit is switched from ON state to OFF state, the terminal voltage at the current-conducting terminal of the main switch circuit is fluctuated by way of the resonance of the resonant capacitor and the magnetizing inductance of the transformer. The main switch circuit is connected to the primary winding of the transformer, and the control terminal of the main switch circuit is connected to the switching control circuit. The switching operation of the main switch circuit is regulated by the control signal outputted from the switching control circuit, such that the energy of the input AC voltage received by the primary winding of the transformer is transferred to the secondary winding of the transformer by way of electromagnetic induction. Thus, an AC voltage is induced across the secondary winding of the transformer. The switching control circuit is also connected to a current-conducting terminal of the main switch circuit in order to detect the terminal voltage of the connected current-conducting terminal. When the main switch circuit is switched from ON state to OFF state and the switching control circuit detects that the terminal voltage is higher than a specific voltage, the switching control circuit turns on the main switch circuit again to allow the main switch circuit to switch under zero-voltage conditions. In addition, the switching control circuit is preset with a maximum OFF time. When the main switch circuit is switched from ON state to OFF state but the switching control circuit can not detect the terminal voltage at the current-conducting terminal of the main switch circuit to turn on the main switch circuit again, the switching control circuit turns on the main switch circuit again in a passive way as the OFF time of the main switch circuit reaches the predetermined maximum OFF time.
FIG. 1 shows the signal waveforms and voltage waveforms of the input AC voltage, the control signal outputted from the switching control circuit, and the terminal voltage at the current-conducting terminal of the main switch circuit. As shown in FIG. 1, when the main switch circuit is switched from ON state to OFF state at the time t1, the terminal voltage of the main switch circuit is fluctuating between the time t1 and the time t2 as a result of the resonance of the resonant capacitor and the magnetizing inductance of the transformer. When the switching control circuit detects that the terminal voltage between the time t1 and the time t2 is higher than a specific voltage, the switching control circuit changes the level of the control signal at the time t2 and thus turns on the main switch again.
Although the switching control circuit of the conventional AC/DC converter is able to turn on the main switch circuit again by detecting if the terminal voltage at the current-conducting terminal of the main switch circuit is higher than a specific voltage, the magnitude of the terminal voltage corresponds to the magnitude of the energy stored in the primary winding of the transformer as the main switch circuit is turned on, and the magnitude of the energy stored in the primary winding of the transformer as the main switch circuit is turned on corresponds to the magnitude of the input AC voltage. If the input AC voltage is too small, i.e. the input AC current is too small, the energy stored in the primary winding of the transformer as the main switch circuit is turned on will be less. If the input AC voltage is too small as the main switch circuit is switched from ON state to OFF state at the time to, the terminal voltage at the current-conducting terminal of the main switch circuit will not be higher than the specific voltage as the input AC voltage is too small. Under this condition, the switching control circuit is not able to detect the terminal voltage and turn on the main switch circuit again. Thus, the switching control circuit is allowed to turn on the main switch circuit again in a passive way only when the OFF time of the main switch circuit reaches the predetermined maximum OFF time, e.g. the period of Ton. Hence, the switching control circuit of the conventional AC-DC converter is not able to control the switching operation of the main switch circuit as the input AC voltage is too small.
To address the aforementioned problems that the switching control circuit of the AC-DC converter is not able to control the switching operation of the main switch circuit as the input AC voltage is too small, some AC-DC converter is configured to allow the switching control circuit to shut down as the input AC voltage is too small. Nonetheless, such AC-DC converter will possess small conduction angle, which in turn result in higher operating current for the AC-DC converter. Hence, the circuit elements of the AC-DC converter, e.g. the resonant capacitor, the main switch circuit, and the transformer, are likely to undergo strong voltage stress and become vulnerable.
Also, some contemporary AC-DC converters are configure to extend the ON time of main switch circuit as the input AC voltage is too small in order to ensure the energy stored in the primary winding of the transformer is sufficient as the main switch is turned on. Thus, the terminal voltage at the current-conducting terminal of the main switch circuit is able to exceed the specific voltage as the main switch circuit is switched from ON state to OFF state, thereby driving the switching control circuit to turn on the main switch circuit again. Nevertheless, the switching control circuit of such AC-DC converter requires sophisticated electronic elements and structures. In this way, the manufacturing cost of the AC-DC converter will be boosted. Furthermore, such AC-DC converter will be have a reduced switching frequency as the input AC voltage is too small due to the extension of ON time of the main switch circuit. As an example, the switching frequency of the main switch circuit can be reduced to 20 kHz. Thus, the AC-DC converter is likely to produce noises as the input AC voltage is too small.